Abstract: Appropriate development of the retinal vasculature is essential for vision. Moreover, abnormal blood vessel growth in diseases such as diabetic retinopathy, age related macular degeneration and retinopathy of prematurity are common causes of blindness. The Retinal Determination (RD) cascade is an evolutionarily conserved signaling pathway best studied in the context of fly eye development and includes proteins of the Eyes Absent (EYA) and Sine oculis homeobox (SIX) families. We have recently demonstrated that Eyes Absent plays an essential role in vascular development. We now have evidence that the SIX family of transcription factors mechanistically synergize with EYA, and that the SIX proteins in both macrophages and in endothelial cells participate in regulating angiogenesis. This competing renewal application builds upon these observations to specifically test the role of the SIX proteins in developmental and pathological retinal angiogenesis. We hypothesize that SIX1 in retinal myeloid cells upregulates VEGFC expression and thus promotes tip-to-stalk conversion and anastomoses in the adjacent retinal vasculature. SIX1/2 in endothelial cells promotes branching and proliferation during tubulogenesis as well as upregulating VEGFC expression, thus reinforcing the tip-to-stalk conversion. To test this hypothesis in vitro and in vivo we propose the following Specific Aims: (I) To elucidate the role of macrophage SIX1 in regulating retinal angiogenesis, and (II) To elucidate the role of endothelial SIX1/2 in retinal angiogenesis. Each aim will be accomplished by integrating diverse techniques including mechanistic biochemistry, cell culture based assays, chemical biology and animal models of eye development and disease. RELEVANCE. The tangible outcomes of this project will include (1) the delineation of mechanisms by which retinal angiogenesis is regulated, (2) the identification and molecular characterization of a pathway that could be targeted for therapeutic benefit in proliferative retinopathies, and (3) the validation of lead small molecule inhibitors that could be further developed as targeted therapeutics.